1. Field of the Invention
The present invention relates to a drop-on-demand inkjet recording device and particularly to a high-speed line-scan inkjet recording device with an ink refresh function.
2. Description of Related Art
There are continuous type and drop-on-demand type inkjet recording devices. Although continuous type inkjet recording devices constantly eject ink from all nozzles, drop-on-demand inkjet recording devices eject ink droplets only as needed. Sometimes nozzles of drop-on-demand inkjet recording devices will not be fired for long periods during printing. Because inkjet recording devices mainly use water-based ink, whose main component is water, the water-based ink near the opening of non-firing nozzles can evaporate and cohere during such long non-firing periods. Once ink is ejected, the poor condition of the ink in the nozzle can adversely affect ejection performance. In bad situations, the nozzle can be completely clogged by the evaporated or cohered ink so that ejection becomes impossible.
Japanese Patent-Application Publication No. SHO-57-61576 discloses a method of vibrating ink to prevent clogging. During periods of non-ejection, the piezoelectric elements for ejecting ink are applied with a smaller energy than required for actually ejecting an ink droplet. This vibrates the ink near the opening of nozzles so that the ink is less likely to cohere. Therefore, vibrating ink can prevent nozzle clogs without increasing consumption of ink. However, merely vibrating the ink does not prevent the water component of the ink from evaporating. When the ink near the nozzle opening evaporates, the viscosity of the ink increases so that ejection performance can be poor. For example, ejected ink droplets may follow a curved trajectory instead of a desirable straight trajectory. Nozzles can also clog up so that ink ejection is impossible.
Japanese Patent-Application Publication No. HEI-9-29996 discloses performing an ink refresh operation in addition to ink vibration. During the ink refresh operation, recording operations are temporarily stopped, the recording head is moved to a predetermined position that is outside the printing range, and then ink is ejected from all of the nozzles in the head. Overly viscous or partially cohered ink near the opening of the nozzles is discharged with the ink ejection and replenished with fresh ink. This method is superior to vibrating the ink in terms of effectively maintaining ejection performance.
Line scan inkjet recording devices are also known in the art. Conventional line scan inkjet recording devices include a print head with an array of nozzles that extend across the entire width of a recording sheet. Line scan inkjet recording devices can record images at high speed because there is no need to transport the print head across the surface of the recording sheet In its widthwise direction. That is, the recording sheet needs to be merely transported continuously in front of the nozzles. However, whenever a refresh operation is performed, recording operations must be temporarily stopped and the print head is moved to a non-printing region. This reduces the recording speed. Further, a complicated mechanism is required for temporarily stopping sheet transport in this way.
Japanese Patent-Application Publication No. 2002-36566 discloses a deflection-type drop-on-demand inkjet recording device that is capable of performing refresh operations without the need to temporarily stop recording operations and move the print head out of the printing range. The nozzles of the print head are divided into groups of 128 to 1,024 nozzles. When there is a period when none of the nozzles in one of the groups is required for image recording, then all of the nozzles in the group are fired together in a refresh operation. The refresh droplets are charged by an electric field and then deflected by a deflection field away from the recording sheet toward an ink collection unit, where the refresh ink droplets are collected.
However, a refresh operation cannot be performed on any group of nozzles as long as even a single nozzle of the group is being used for image recording. When printing a vertical straight line or other image that is elongated in the transport direction of the recording sheet, then refresh operations cannot be performed for long periods of time on nozzle groups with nozzles used in the elongated image. Nozzles of such groups that are not used to record the image will have problems described above such as ink cohering so that ink ejection is defective or impossible.
To prevent such problems, it is conceivable to provide an ink refresh ejection period in addition to recording ejection periods. The ink refresh ejection period is used solely for ink refresh operations. In general, a time-sharing method is used wherein an ink refresh ejection period is interposed between two consecutive ink recording ejection periods. In order to reduce ink consumption, the fewer times that ink refresh is performed the better. It has been determined by tests that, under normal environmental conditions of temperature and humidity, sufficient effects are achieved by performing refresh operations at a frequency of only 10 Hz-20 Hz.
This type of refresh operation is well suited for low-speed recording devices, but not very well suited for high-speed recording devices, such as line scan inkjet recording devices. Normally recording at high speeds is achieved by electing droplets at a high ink ejection frequency f. However, in order to eject an ink droplet, each voltage drive signal that is applied to a piezoelectric element to eject an ink droplet needs to be applied for a certain time duration, for example, 80 micro seconds as shown in FIG. 1(a). This time requirement for duration of the drive signal limits the frequency that signals can be applied. For example, when the drive signal must be a minimum of 80 micro seconds long, then the drive signals cannot be applied at a frequency of greater than 10 kHz, so the maximum ejection frequency fm (Hz) is 10 kHz.
At this time, the speed at which a recording sheet can be transported, that is, a sheet transport speed Vp, can be represented using the following formula:Vp=f/R  (1)
wherein f is the ejection frequency; and
R is the resolution (in dots/inch) in the sheet transport direction.
For example, the maximum sheet transport speed Vpm is 33.3 inches/second for printing an image with a resolution of 300 dpi (dots/inch) at the maximum ejection frequency fm of 10 kHz.
However, when recording is performed at a high speed near or at the maximum ejection frequency fm of 10 kHz, only a short interval separates successive drive signals as shown in FIG. 1(a). In this case, there is insufficient time for also outputting an ink refresh drive signal. A longer interval between successive drive signals is required if the time-sharing method is to be used.
However, normally both the recording resolution and sheet transport speed are maintained constant to facilitate synchronization of ink ejection and sheet transport operations. Therefore, the duration of each drive signal is also constantly the same. Accordingly, the interval between successive drive signals cannot be temporarily lengthened only at certain times. Therefore, even if ink refresh operations are performed only very infrequently, the interval between successive drive signals must be increased for all drive signals as shown in FIG. 1(b). As a result, in order to enable refresh operations during printing operations, the actual ejection frequency f must be met to half the maximum ejection frequency fm of 10 kHz or less, that is, to 5 kHz or less.
Naturally, the recording speed Vp also decreases. That is, from formula (1) it can be understood that:Vp=f/R=16.7 inches/second  (2)
The sheet-transport speed also drops by half or less. This creates a big problem when attempting to produce a high-speed recording device.